1. Field of the Invention
This invention relates to nuclear radiation, and in particular, relates to the production of a photon beam having a single high energy which is tunable.
2. Description of the Related Art
Gamma rays are high energy photons, generally emitted from radioactive nuclei. Various studies have been done in an attempt to produce a tunable monoenergetic photon beam. For example, Lindenstruth et al., Nuclear Instruments and Methods in Physics Research, Vol. A300:293 (1991), performed nuclear resonance fluorescence experiments on selected radioactive isotopes to determine the relative spectral shape of low energy thick target bremsstrahlung spectra. The disclosure of this paper and of all other papers and patents cited herein is incorporated herein by reference. The emission of bremsstrahlung by charged particles in flight is limited in that this radiation has the characteristic of a continuous electromagnetic spectrum, and the photon energy ranges from zero to a finite limit. To obtain a monoenergetic beam using bremsstrahlung spectra, radiation of a particular bandwidth would need to be tagged and sorted out, requiring complicated electronic procedures and resulting in a low gamma ray yield per particle of the incident beam. This process is cumbersome and results in many "noise" photons at undesired energies, and large bandwidth of the selected photon beam.
Other researchers have attempted to utilize Compton scattering of laser photons from an incoming beam of highly-relativistic particles to develop a tunable gamma ray source. Thus, Thorn et al., Nuclear Instruments and Methods in Physics Research A285:447 (1989), described a spectrometer for tagging a gamma ray beam produced by Compton backscattering laser light from an electron beam circulating in a storage ring.
Satisfactory tunable monoenergetic beams of photons have therefore not been produced, and there has been no known way to obtain such a monoenergetic beam with a variable energy. Such beams have many potential uses, for example, in medicine to determine the presence of a particular element in the body without over-exposing the body to a multitude of extraneous wavelengths of radiation.
Single quantum annihilation of positrons is recognized as a fundamental electrodynamic process of atomic physics, but not many studies have been made of this phenomenon. Thus, when a positron, which does not decay spontaneously, passes through matter it sooner or later collides with an ordinary electron. In this collision, the positron is annihilated, and the total energy of the positron and the electron is converted into electromagnetic radiation in the form of one or more photons.
Single quantum annihilation of a positron in flight with a bound atomic electron takes place in the Coulomb field of the nucleus. The K-shell of the atom is responsible for about 80% of the annihilation, with electrons in the higher shells contributing the rest of the annihilation. The energy of the photon is given by E.sub..gamma.=E+ 2Mc.sup.2 -B, with mc.sup.2 representing the rest-mass energy of the electron/positron, and B the binding energy of the atomic electron that is annihilated with the positron.
For any particular atomic shell, it is postulated that the photons obtained by annihilation are highly monoenergetic, the energy width being accounted predominantly by the energy divergence of the incident positrons and the spread introduced as the positrons traverse the target material. The net energy width of the emitted photons, in terms of the incident-energy divergence .delta.E of the positrons and the thickness .mu. of the target, is given as: EQU .delta.k={.delta.E.sup.2 +(.mu.dE/d.mu.).sup.2).sup.1/2,
where dE/D.mu. represents the energy deplection rate of the positrons for transmission in the target material, which is typically around 1 keV//(mg/cm.sup.2). Previous work, however, has not yielded a method or apparatus for producing monoenergetic photons, the energy of which is tunable (adjustable in energy).
It is therefore an object of this invention to provide an apparatus and method for producing a tunable source of monoenergetic photons.
It is a further object of this invention to provide means of producing a beam of protons which is highly directional and forward peaked (is self-collimated), and is polarizable.
It is a further object of this invention to provide a means of producing a beam of photons which can have a very narrow width, which can be regulated by regulating the width of an incident beam of positrons and the thickness of the target.
It is a further object of this invention to provide a means of producing a beam of photons with an intensity which is high in proportion to the intensity of the incident positrons as compared to previous methods of obtaining monoenergetic photons of variable energy.
It is another object of this invention to provide a source of monoenergetic photons which does into depend on the use of a tagging device.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.